Wireless LAN Performance Analysis

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Wireless LAN Performance Analysis
Presented By Nikos Ioannou
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Wireless LAN

• Architecture
• Access Points
• Clients
• Types of wireless LAN
• Peer-to-peer
• Bridge
• Wireless Distribution System

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Wireless LAN
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IEEE Standards

• 802.11b (Wi-Fi)
• 802.11a
• Both define specific physical (PHY) layers

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IEEE Standards (Summary)
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Performance Analysis in Wireless LANs

• WLAN Propagation Overview
• Available Spectrum and Efficiency
• Signal Range Overview
• Power Considerations Overview
• Throughput Overview
• Metrics
• Performance analysis using reference model
• Performance analysis using measured data

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WLAN Propagation Overview

• Three primary modes for Electromagnetic signal
• Reflection
• Diffraction
• Scattering
• For indoor environment the signal does not
  predictably lose energy so is difficult to
  calculate Path Loss.

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Available Spectrum

• IEEE 2.4 GHz 802.11b
• Uses DSSS Modulation
• Requires 22MHz band for one network
• With 83.5 MHz band will supports 3
  non-overlapping simultaneously Wi-Fi Networks

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Available Spectrum (cont)

• IEEE 5GHz 802.11a
• Uses OFDM Molulation
• Requires 16.6 MHz band for one network
• With 300MHz of spectrum supports 12
  non-overlapping simultaneously networks

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Signal Range Overview

• The signal range of every wireless system is
  governed by the following variables
• RF power transmit level
• Required Es/N0 (Signal Energy required to
  recover the transmitted symbol)
• Environment
• Signal Propagation

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Signal Range Overview (cont)

• One of the fundamental differences between
  communications operating at 2.4 and 5GHz is the
  achievable communication range between the AP and
  the station. Holding variables above constant 2.4
  GHz frequency offer roughly double the range of
  those operating in the 5GHz band

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Signal Range Overview
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Power Considerations

• The limitation in range is caused by the more
  severe path loss of the 5GHz spectrum
• By increasing the power of a 5GHz system
  approximately 4 times can achieve ranges similar
  to 2.4GHz systems
• 802.11a system (5GHz) is more power efficient
  over small areas and 802.11b(2.4GHz) is more
  efficient over greater distances

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Throughput Overview
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Metrics

• Range is the greatest distance from an AP at
  which the minimum data rate demodulated with an
  acceptable BER
• Coverage is the resulting cell size or square
  meters per AP
• Rate weighted coverage is the bit rate with
  respect to area covered or (Mb/secm²)
• Transmit Power upper bound (for 802.11a 16.02dBm
  in 5.15-5.25GHz, 23.01dBm in 5.25-5.35GHz and for
  802.11b maximum of 30dBm)
• Receiver Sensitivity
• Noise and Interference (ACI,CCI)
• Bit error Rate

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Receiver Sensitivity

• For IEEE 802.11b receiver should be able to
  detect -76dBm with BER of min 10e-5 in the
  absence of ACI. If ACI is present the receiver
  must be able to detect -70dBm
• For IEEE 802.11a as follows

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Bit Error Rate

• IEEE 802.11b for BER better than 10e-5 then min
  S/N
• IEEE 802.11a for BER better than 10e-5 then min
  S/N

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The Model

• For indoor environment the signal power at the
• receiver SRx is related to the transmit power
• TRx as shown below (this model will be used as
  the reference analysis model)
• Where Cspeed of light, fcenter frequency, N
  path loss coefficient. ITU recommends N3.1 for
  5-GHz and N3 for 2.4-GHz

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Analysis using reference model (Range
Comparisons)

• IEEE 802.11b (with N3)
• With EIRP of 30dBm max range154m
• With EIRP of 19dBm max range66.4m
• With EIRP of 15dBm max range48.4m
• IEEE 802.11a (with N3.1)
• With EIRP of 18dBm range14m with 54Mbits /s
• With EIRP of 23dBm range30m with 54Mbits/s

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Analysis using reference model (Practical
Deployments)

• To cover greater areas networks designers deploy
  multiple APs in a hexagonal cellular arrangement
  (like cellular telephony)
• We assume also that only one station is
  transmitting in each cell
• Spectral leakage (l) from one adjacent channel
  is -23dB for a 802.11a network and
• -34.40dB for a 802.11b network

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Analysis using reference model (Practical
Deployments 3cell net)

• For 802.11b (we have only 3 frequency channels)
  one of the 3 cells experiences ACI from the other
  two cells. The greatest interference noise power
  (IN) is experienced by a user located at point G.
  Desired signal power is P(Rx). Therefore SINR(2f
  )-1 which is 31.29dB means all data rates are
  supported.
• The size of the network below can be calculated
  by finding the radius of each hexagon. Using the
  min Rx sensitivity of -70dBm an 15dBm solution
  can cover 7397m² at 11Mbs while a 19dBm solution
  13670m²

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Analysis using reference model (Practical
Deployments 3cell net)

• For 802.11a the designer can choose channels
  1,5,8 for cells I,II,III so there is no ACI. With
  EIRP 23.01dBm for cells I and III and 16.02dBm
  for cell II and min Rx sensitivity we have a
  total coverage of 2730m² at 54Mbits/s (as the
  required rate)
• Alternatively if the required rate is 12
  Mbits/sec with EIRP 23.01dBm the total area
  covered becomes 17650m² (SINRf-1)

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Analysis using reference model (Practical
Deployments 8cell net)

• In an 8-cell 802.11a network all APs transmit at
  the same power of 16.02dBm and the greatest
  interference is experienced by a station
  belonging to cell IV located at G. The sources of
  interference are the two ACs one at 2R and the
  other at root 7R. The SINR is given below giving
  about 31.49dB means that a rate of 54Mbs is
  accessible throughout the network. The total area
  covered is 2082m².

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Analysis using reference model (Practical
Deployments 8cell net)

• Similarly a 802.11b can be used to cover the same
  geometry. Again the most interference is
  experienced by a station located at vertex G.
  This leads to co-channel at 2R, a co-channel at
  root 7R, two adjacent at 2R, two adjacent
  channels at R and one adjacent channel at root
  7R. Therefore the least SINR is shown by the
  equation below or about 7.45dB. With this result
  the 11Mbits/s is accessible throughout the
  network. A 15dBm solution can cover 19727m² while
  a 19dBm will cover 36453m²

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Performance analysis using Measured Data

• The measured performance data collected in a
  typical office environment 265 foot by 115 foot
  rectangular with conference rooms closed offices
  and walls as well as semi-open cubible spaces.
• For the 802.11a system data was sent between two
  PC cards one as fixed AP and the other as a
  mobile station with output power of 14dBm.
• For the 802.11b system comprised as AP and a PC
  card with output power of 15dBm

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Data Link Rate results (for test environment with
one AP and a mobile Station)
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Throughput Results (for test environment with one
AP and a mobile station)
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System Capacity under CCI using measured data

• The analysis will be for the following 8-cell
  systems
• System Capacity refers to the throughput of an
  entire WLAN comprised of many cells

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System Capacity under CCI using measured data

• Co-Channel interference is less for 802.11a
  systems than 802.11b due to the presence of more
  channels

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Average Cell Throughput under CCI using measured
data

• Measured performance data from single AP-User
  inputted into a system capacity model proposed by
  NEC1 to evaluate the system capacity of the
  8-cell WLAN system above

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System Capacity under CCI using measured data

• System capacity as the average cell throughput
  (shown above) multiplied by the number of cells.

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